1. Field of the Invention
The present invention relates to a device and a method for processing of semiconductor wafers and fabrication of microelectronic devices.
2. Description of the Relevant Art
Advanced semiconductor VLSI technologies employ plasma processing for a number of important steps in the device fabrication sequence. The use of plasma processing results in a lower processing temperature and a higher deposition rate for growth and deposition of thin layers of insulators, semiconductors, or metals. Moreover, reactive ion etching processes in low-pressure plasmas are essential for anisotropic patterning of the submicron features in VLSI device structures.
Conventional plasma processing techniques, however, suffer from a number of limitations. Low-pressure reactive ion etching systems with RF (usually 13.56 MHz) discharge offer relatively low gas ionization efficiencies and can cause damage to the devices due to the large substrate floating potentials and excessive ion energies and plasma deposition processes using conventional discharge methods can also suffer from similar problems. A major disadvantage of the conventional RF discharge techniques is that the average ion energy cannot be controlled or adjusted independently and any adjustment of the ion energy results in a change in the plasma density and, as a result, the process kinetics.
Generation of plasmas by electrodeless microwave (2.45 GHZ) discharge is a viable alternative to the conventional RF discharge methods. The use of microwave power instead of lower frequency RF power enhances the gas ionization efficiency and reduces the ion bombardment-induced damage to the wafer due to the lower iow acceleration potentials. The coupling of microwave power to the discharge medium can be made very efficient if a suitable static magnetic field is established in the discharge medium. Under the so-called electron cyclotron resonance (ECR) condition, the electrons in the plasma experience a cyclotron (spiral) motion around the static magnetic flux lines while gaining energy from the alternating microwave eletric field. In a uniform magnetic field, the cross section of the path traversed by the electrons is a circle. The frequency of the cyclotron rotation (f.sub.c) is directly proportional to the magnetic flux density (B) and inversely proportional to the electron mass (m.sub.e), as shown below: ##EQU1## where q is the electronic charge. Based on this equation, the frequency of cyclotron rotation is calculated to be ##EQU2## in cycles per second where B is the static magnetic flux density in Gauss. Under ECR condition the electron cyclotron rotation frequency becomes equal to the microwave frequency. For a microwave frequency of 2.45 GHz, the ECR condition requires a magnetic flux density of 875 G. It should be noted that the cyclotron frequency only depends on B and is independent of the electron velocity. The curvature radius of the electron spiral path is inversely proportional to the component of the magnetic flux density perpendicular to the electron velocity vector.
The average power transferred to each electron in a microwave field is maximized when the ECR condition is satisfied because the cyclotron motion and the microwave electric field oscillations stay in phase with each other. From an electrical point of view, the plasma impedance is highly reactive (inefficient power absorption) at high frequencies. In an ECR microwave plasma, an electron continuously absorbs energy from the microwave field until it experiences a collision with a neutral gas species. Higher gas pressures result in larger electron collision probabilities and less energetic electrons in an ECR plasma because of a more frequent disturbance of the electron free spiral motion. As a result, ECR plasma processing effects will be more pronounced when the electron-neutral collision frequency is made much less than the ECR or microwave frequency. This implies that ECR is particularly useful for lower pressure processing (few mTorr and below; 10.sup.-3 -10.sup.-5 Torr range) where conventional RF discharge is rather inefficient.
ECR plasma generation techniques are capable of producing efficient plasmas at low pressures with much higher densities (as much as several orders of magnitude) compared to the conventional RF discharge or non-ECR microwave plasma techniques. The ECR enhancement also extends the operating process pressure domain down to very low pressures in the high-vacuum regime. ECR plasma processing is applicable to a wide range of advanced semiconductor device fabrication processes (dry clean-up, deposition, etching) as well as sequential in-situ mutiprocessing.
Most of the existing ECR plasma system designs employ electromagnets in order to generate a static magnetic field inside the plasma formation chamber. FIG. 1 shows the general schematic of an ECR plasma processing system which employs two electromagnets around a microwave discharge cavity in order to establish a spatially varying magnetic flux density and generate and ECR plasma stream. The electromagnets create a graded magnetic field inside the microwave discharge cavity and the ECR field condition (875 G) is satisfied at some point inside the cavity. The plasma stream is extracted along a divergent magnetic field from the plasma chamber to the reaction chamber. The magnetic flux density decreases gradually from the plasma chamber towards the substrate holder.
This type of ECR plasma system design has a number of limitations which can be summarized as follows:
The process uniformity on the wafer is very sensitive to the plasma uniformity in the plasma chamber and the plasma cavity electromagnetic mode of operation (standing wave patterns). The plasma nonuniformity patterns can be easily projected onto the wafer because of the presence of longitudinal magnetic field lines.
The system design is not easily scalable for larger wafer diameters. Larger wafers dictate the use of larger plasma cavities and larger electromagnets which can result in a less uniform plasma and a more complicatd reactor design.
The diverging magnetic flux lines extend all the way to the wafer surface and result in a less uniform process even if perfect plasma uniformity is achieved in the plasma cavity. The process nonuniformity problems caused by the field divergence effects are particularly more critical in dry etching applications.
The large electromagnets require water cooling and a large amount of electrical power to sustain the magnetic field.
Precise ion energy control is difficult because the wafer experiences perpendicular magnetic field lines and the ions gain translational acceleration by moving along the high-to-low magnetic flux lines extending from the plasma chamber to the wafer. The field lines can also affect the plasma electrical potential.
In some existing ECR reactor designs the process uniformity is somewhat improved by using a third electromagnet under the wafer holder in order to produce a more uniform perpendicular magnetic field and reduce its divergence on the wafer. Moreover, the substrate holder may be coupled to an RF source (13.56 MHz) in order to control the wafer potential with respect to the plasma to enhance the incoming ion energies and to reduce the divergent magnetic field effects. Nonetheless, these designs do not remove the other limitations of this type of reactor design.
Besides the ECR reactor designs similar to the schematic shown in FIG. 1, a multipolar distributed ECR reactor has also been discussed in the literature. FIG. 2 shows the schematic of a multipolar ECR equipment design where the energetic electrons are confined to the magnetic cusps at the outer edge of the process chamber. In this system, plasma is produced near the chamber wall and diffuses towards the chamber center. The magnetic field lines at the center of the chamber are rather weak and parallel to the wafer surface. The multipolar field created by the permanent magnets creates the ECR condition and reduces plasma losses to the chamber walls by magnetic confinement of the plasma. The multipolar ECR design may have some advantages over the conventional ECR systems (and also some disadvantages). Some of its limitations are as follows:
The plasma formation chamber is the same as the process chamber. This restricts the system applications to the processes with one composite plasma medium. Any gas injected into the vacuum chamber will be subjected to microwave discharge and the system design does not allow selective plasma formation simultaneous with injection of non-plasma gases onto the wafer. Therefore, the system application will be mostly for etching processes.
The entire process chamber and the wafer are immersed in a microwave field. As a result, the plasma and process uniformity on the wafer may be affected by the microwave standing wave and power absorption patterns.
The total volume of the ECR plasma formation regions is a small fraction of the total process chamber volume. This may limit the plasma density on the wafer.
Accordingly it would be useful to be able to provide a distributed ECR device that has process uniformity, is scalable, has low power consumption, causes less substrate damage, has an independent non-plasma gas injection capability provides a remotely generated ECR plasma allows independent control over plasma density, and allows sequential in-situ multiprocessing.